burkhardt etal



Oct- 1, 1963 G. BURKHARDT ETAL 3,105,908

PHOTOELECTRIC SOR'IING METHOD AND APPARATUS File y 19, 1960 2Sheets-Sheet 1 AMPLITUDE m mvsurons +5 Gisbert Burkhordtfi:

FIG 3; Kurt Kirsch BY W ATTORNEY 1963 G. BURKHARDT ETAL 3,105,908

PHOTOELECTRIC SORTING METHOD AND APPARATUS Filed July 19, 1960 2Sheets-Sheet 2 FIG.4.

G' b '8 W??? IS er ur or H G Kurt Kirsch BYM ATTORNEYS 3 Germany UnitedStates Patent 3,105,908 PHOTOELECTRIC SORTING METHOD AN APPARATUSGisbert Burkhardt and Kurt Kirsch, Konstanz (Bodensee), Germany,assignors to Telefunken G .m.b.H., Berlin,

Filed July 19, 1960, Ser. No. 43,843 Claims priority, applicationGermany July 25, 1959 15 Claims. (Cl. 250-219) The present inventionrelates to a method and apparatus for sorting mail or the like. Moreparticularly, the present invention relates to a method and apparatusfor sorting mail carrying fluorescent markings.

, In mailsor ting apparatus, it is advantageous to have some marks ormarkings on the mail itself, which marks or markings correspond to thedestination to which the mail is going, so that the physicalcharacteristics of the markings can be sensed by means of magnetic orphotoelectric sensing apparatus. The markings serve as a guidingcriterion so that the process of distribution is actually controlled bythe mail itself.

, In some conventional apparatus, the marks on the mail, which may be inthe form of letters, postal cards or parcels, are visible to the nakedeye. The sensing means then must sense these marks in the visible range.However, most items of mail, have on either the front or back thereof asubstantial number of markings which are in the optically visible range.For example, the address and other similar material often appears on theouter portion of letters. Thus, a mail-sorting apparatus can be misledif guide-marks of the above mentioned kind are used.

. An alternative which might occur to one skilled in the art is theutilization of magnetic markings. However, surprisingly, this methoddoes not work too well, because much of the mail distributed todaycontains some ferromagnetic ma-terial of one kind or another which maycause the mail-sorting apparatus to make errors in indicating thedestination of the mail. For example, steel staples are commonlyusedlwhich may cause the magnetic mail-sorting apparatus to make errors.Furthermore, the magnetic markings are generally opaque and thereforecan impair the legibility of, e.g., the address if this is not properlyarranged.

In accordance with the present invention, the markings are made from afluorescent material which is transparent, or at least translucent.These markings do not become-visible until they are irradiated withultraviolet rays. Accordingly, an appropriate ultraviolet source isutilized and a photoelectric apparatus is provided for receiving therays from the fluorescent material. This serves to avoid thedifficulties caused by optically visible markings and magnetic markings.It has been found, however, that some of the paper used in letterenvelopes and wrapping paper for parcels contains material which alsorespond to ultraviolet rays, i.e., materials which become visible whenirradiated with ultraviolet light. Accordingly, it is an object of thepresent invention to provide a new and improved method and apparatus forsorting mail, such as letters, postal cards, parcels and the like, saidmethod and apparatus overcoming the above mentioned and otherdisadvantages of the heretofore known methods and apparatus.

It is another object of the present invention to provide a method andapparatus for sorting mail and the like, using fluorescent markings onthe mail itself and a photo electric apparatus for detecting anyemissions from the fluorescent material, reliably discriminating betweenthese emissions and interfering emissions from the wrapping paper.

It is an additional object of the present invention to provide a new andimproved apparatus for sorting mail, said apparatus being of ruggedconstruction, can be built at low cost and will give long periods oftrouble-free and error-free service.

With the above objects in view, the present invention resides mainly ina method of an apparatus for automatically sorting mail, the methodincluding the steps of depositing a fluorescent material on the mail tobe sorted, the material having such properties that a substantialportion of the energy emitted therefrom upon excitation includes rayshaving wave lengths greater than 550 milli microns, and producing alonga preselected path exciting rays for inducing fluorescence of thefluorescent material, substantially all of said exciting rays havingwave lengths in the region between 300 and 400- millimicrons. The mailto be sonted is then moved into the preselected path of the excitingrays, whereby these rays impinge upon the fluorescent material. Thus,any rays emitted from the fluorescent material upon excitation aredetected which have wave lengths larger than 550 millimicrons, therebyindicating the exact nature of the markings on the mail.

In apparatus operating in accordance with the principles of the presentinvention, the mail is marked with a fluorescent material emitting raysupon excitation comprising wave lengths longer than 550 millimicrons.Means are provided for producing exciting rays for the fluorescentmaterial along a preselected path, substantially all of the excitingrays having wave lengths in the region between 300 and 400 millimicrons.Means are provided for moving the mail to be sorted into the preselectedpath so that the exciting rays impinge on the fluorescent material andadditional means are provided for detecting any rays which are emittedfrom the fluorescent material due to the impingement of the excitingrays thereon, said detecting means being responsive only to rays longerthan 550 millimicrons.

According to another feature of the present invention, a low-pressuremercury vapor lamp is used as the source of the exciting rays andtransforming means are provided for transforming the 254l'l'lll'llll'l'llCl'OIl emission of this lamp into exciting rays havingwave lengths longer than 254 millimicrons.

Additional objects and advantages of the present invenf tion will becomeapparent upon consideration of thefollowing description when taken inconjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic arrangement of an apparatus incorporating theprinciples of the" present invention.

FIGURE 2 is a graph showing normalized curves of emissivity andfilter-transmissivity which are referred to in explaining the presentinvention.

FIGURE 3 is a schematic illustration, partially in section, of anotherembodiment of the present invention.

FIGURE 4 is a schematic view similar to FIGURE 1 illustrating anotherembodiment.

FIGURE 5 is a partial sectional view taken substantial'ly along theplane generally defined by reference line s s of FIGURE 3. I

Refer-ring in detail to the drawings, and more particularly to FIGURE 1thereof, it will be seen that a letter or other item of mail designatedby the numeral 10 is moved along the flat surface 11a of a member 11 inthe direction of the arrow 12. The mail 10 carries fluorescent marks 13which are approximately equal to the size of an opening 14 in thememberll. The term mail or item of mail, as used throughout the instantspecification 'and claims, is deemed to include not only any letter,postor postal card, printed matter, parcel, or the like, customarilyhandled by postal authorities, but any item or article which PatentedOct. 1, 1963- is conveyed and which must be routed to a certaindestination.

Arranged on the opposite side of the member 11 is a source of excitingrays 16 which emits rays along a path outlined by the dashed lines 17.This path intersects the opening 14 in the member 11. The exciting rayspass through a filter 18 arranged between the member 11 and the source16. Spaced from the member 11 is a photoelectric detecting member 19having terminals 20, 21, adapted to be connected to an amplifier andindicator or the like (-not shown). A second filter 22 may be arrangedbetween the photoelectric detecting apparatus 19 and the member 11.

In operation, the mail to be sorted is moved along the flat surface 110of the member 11 in the direction of the arrow 12. The exciting raysproduced by the source 16 are transmitted through the filter 18 and passthrough the opening 14 so as to impinge on any fluorescent markings 13carried by the mail 10. Should the exciting rays impinge on afluorescent marking, this fluorescent material will emit rays which passthrough the filter 22, said rays being detected by the detectingapparatus 19. This detecting apparatus 19 can then direct the routing ofthe mail as it passes beyond the member 11 by means of conventionalamplifiers and control devices.

As mentioned above, certain papers used as envelopes or as wrappingpaper for parcels have the property of fiuorescing when irradiated withrays in the ultraviolet region. Accordingly, if such paper is used, itis sometimes difficult for the detecting apparatus to differentiatebetween the rays emitted from the paper and the rays emitted from thespecial fluorescent markings. The use of a filter for differentiatingbetween the different types of rays produced does not work too well,since these filters have transmission characteristics incompatible withconventional type sources of exciting rays. For example, conventionalsource is a high-pressure mercury vapor lamp. One disadvantage of such alamp is that the highpressure mercury vapor lamp is a source of intenseheat which may damage the mail being sorted as well as the sortingapparatus itself. Furthermore, the high-pressure mercury vapor lamp isparticularly sensitive to voltage fluctuations, and a drop in the supplyvoltage, even for a short time, may cause the lamp to be extinguished.As the ordinary high-pressure mercury vapor lamp, once extinguished,will not re-light for a substantial period, a considerable amount ofmail will be misdirected, since without an operative source of excitingrays the detecting apparatus is unable to respond to the fluorescentmarkings showing the destinations of the individual pieces of mail.

The above disadvantages are overcome by providing a low-pressure mercuryvapor lamp. According to the prior art such lamp would be used with anultraviolet filter which has a very good transmission characteristic atthe wave length of 254 millimicrons, which is the wave length of themain spectral line of this lamp. The known filters of this kind however,for instance, the filter known as Schott UG 5, have also a goodtransmission characteristic for wave lengths longer than 65 Omillimicrons which will permit the passage of red rays, as explainedbelow, having a wave length of 691 millimicrons, said rays being pickedup directly by the receiving or detecting apparatus without beingproduced by the fluorescence of the direct markings. This will cause amisdirection of the mail.

To overcome this disadvantage, in accordance with the present invention,the arrangement shown in FIGURE 1 has the following characteristics:

(a) The fluorescent material used for marking the mail has the propertythat, upon excitation, an essential part of its emissive energy isemitted at wave lengths longer than 550 millimicrons.

(b) The source of exciting rays is a low-pressure mercury vapor lamp.Means are provided for transforming the rays produced by this sourcehaving a wave length of 254 millimicrons to ultraviolet rays havinglonger wave lengths. The filter 18, arranged between the source 16 andthe member 11, has a good transmission characteristic for rays havingwave lengths in the region between 300 and 400 millimicrons, while inthe visible range, the filter has a transitivity as low as possible forrays having wave lengths longer than 550 millimicrons.

(c) The detecting apparatus for receiving the rays emitted from thefluorescent material 13 upon excitation thereof has a relatively broadfrequency spectrum, the short-wave limit of its over-all sensitivitybeing in the range of 550 millimicrons.

Referring now to FIGURE 2, the transmission characteristic indicated bythe dot-dash line a refers to the filter Schott UG 5 as mentioned above.It can be seen that this filter has an excellent transmissioncharacteristic for the wave length of 254 millimicrons, this wave lengthcorresponding to the main emission of a low-pressure mercury vapor lampand being shown by the line 24 in FIGURE 2. It should be noted, however,that these filters also have a good transmission characteristic for wavelengths in the red and infrared region, as shown by the second pontionof the dot-dash line a in FIGURE 2. It can be seen that the red spectralline of the mercury vapor lamp having a wave length of 691 millimicronswill pass through this filter, if used as filter 18, quite readily.

In order to prevent the red spectral line passing through this filterfrom being picked up by reflection from the mail 10, whether or not itcarries fluorescent markings, according to prior art the detectingapparatus 19 could be provided with a filter 22, having a transmissioncharacteristic which is shown as a solid line b in FIGURE 2. It will beseen that, in order to attenuate any rays produced by the fluorescentproperties of the paper itself, this filter is a very narrow band-passfilter.

The dashed curve c in FIGURE 2 indicates the frequency spectrum and therelative amplitude of the emissive rays which may be emitted from thepaper used for and/or as the item of mail itself and having fluorescentproperties. The curve c does not represent any particular type of paper,but rather represents an envelope of the fluorescence properties ofdifferent kinds of papers which may be utilized and which the sortingapparatus should be able to distinguish from fluorescent markings. Itcan be seen from FIGURE 2 that by using the narrow band-pass filter ofcurve b the unwanted emission of the paper itself will be partiallysuppressed compared with the emission of fluorescent markings. Theuseful output however is relatively low, resulting in poorsignal-to-noise ratio. As mentioned above, the present invention uses acombination of (a) the characteristics of the fluorescent materialactually marked on the paper for sorting purposes, (b) the source of theexciting rays, and (c) the detecting apparatus itself. According to thepresent invention, the fluorescent material has the property that asubstantial portion of the energy emitted therefrom upon excitationincludes rays having wave lengths greater than 550 millimicrons. It isnot absolutely essential that the rays emitted from the fluorescentmaterial have the largest amplitude in this range, merely that asubstantial amount of energy is emitted in this range. A suitablefluorescent material is known as yellow-orange Lumogen UV having anemission characteristic such as shown in FIGURE 2 by the curve d.

In accordance with features (b) of the present invention, a fluorescentlayer 30, shown in FIGURE 1, is deposited about the low-pressure mercuryvapor lamp, said layer transforming the 254-millimicron-emission of thelamp 16 into an ultraviolet emission of greater wave lengths than thosenormally produced by a lamp of this type. As shown in FIGURE 4 whereinelements similar to those of FIGURE 1 are indicated with identicalreference numerals, a fluorescent layer 30' may be arranged between thelamp 116 and the filter 18. In another embodiment, the fluorescent layer30 might be arranged on the surface of the filter 18, at the side whichfaces the low-pressure mercury vapor lamp 16. 1

Thus, a shift of the spectral energy distribution of the emittedultraviolet rays is accomplished which makes possible the use of afilter 18, according to the invention, the transmission properties ofwhich are shown by curve e in FIGURE 2. One filter of this type is knownas Schott UG 11. It will be seen that at 254 millimicrons thetransmission factor of this filter is poor, but it is also low at wavelengths longer than 550 millimicrons. Maximal transmission takes placein the range between 300 and 400 millimicrons. Similarly, thetransmission characteristic at the red mercury line of 691 millimicronsis substantially lower than that of the formerly mentioned filter UG 5corresponding to curve a. With this arrangement, it is then possible touse a detecting apparatus which according to the invention has a verybroad frequencyresponsive spectrum, the short-wave limit of which shouldbe about 550 millimicrons. This can be accomplished by utilizing as thefilter 22 a filter which has a transmission characteristic shown by thesolid curve 1 in FIGURE 2. Such filters are known as the Schott 0G 2 or0G 3.

The use of a detecting apparatus having a broad frequency spectrum withthe lower limit at 550 millimicrons has several advantages. For example,in combination with the energy distribution of the proposed fluorescentmaterial as shown by the curve d of FIGURE 2 it results in a very goodsignal-to-noise ratio in the receiver. This signal-to-noise ratio isessentially better than the one which would be produced using a filterhaving the characteristics shown by the curve b of FIGURE 2. Thisbecomes evident from FIGURE 2 by comparing the areas belonging to thesignal energy according to curved and to the noise energy according tocurve c, both within each of the broad spectral region of filter curve1'' and of the small spectral region of filter curve b respectively.Furthermore, the large breadth of the frequency spectrum of thedetecting and receiving apparatus allows the absolute amplitude of thesignal energy to be such, that it is well above the background noise ofthe receiver itself.

FIGURE 3 is another embodiment of the present invention, showing theactual structural characteristics of the various components. In thisembodiment, the lowpressure mercury vapor lamp has a U-shaped ray source40, the legs of the U-shaped source being shown in this figure. Themember 41, along which the letters pass as described by explainingFIGURE 1, has branches 41a and 41b provided with openings for receivingand positioning the filters 42 and 43. A further opening 44 permits thepassage of the rays through the member 41. The detecting apparatus 60,which may be constituted by a photo-electric cell, is arranged on thehousing of member 41 and has a filter 51 positioned between theapparatus 60 and the opening 44.

The filter 51 has the characteristics shown in wave form f in FIGURE 2,and the filters 42 and 43 have the characteristics shown by the curve ein FIGURE 2.

When the sensing arrangement is in use, the opening 44 in the wall ofthe member 41 is arranged to correspond to the size of the fluorescentmarks on the mail to be sorted which, for example, may be coded guidingmarks. A further improvement can be added to this arrangement bypositioning in front of the detecting member 60 a translucent member 54serving as light conductor. If, however, the fluorescent material iscontained in the stamps a-fllxed to the mail, opening 44 must beenlarged accordingly.

If several series of fluorescent marks placed side by side are used onthe mail, a plurality of detecting apparatus 60 arranged parallel to oneanother may be used as shown in FIGURE 5. These apparatus are stacked ina direction perpendicular to the plane of the drawing 6 v of FIGURE 3.Light conductors 54 are used, and one conductor is provided for eachdetector 60. The individual-detectors 60 and light conductors 54 areseparated from one another by means of partitions56 which are arrangedparallel to each other. In order to increase the sensitivity of thedetecting apparatus, these partitions may be made of reflectingmaterial.

In operation, the apparatus is similar to that shown in FIGURE 1, theU-shaped low-pressure mercury vapor lamp having coated on its legsfluorescent layers 50 which serve to transform the rays emitted from themercury vapor lamp from about 254 millimicrons to ranges of higher wavelengths. The combination of these fluorescent layers 50 and the filters42 and 43 produce rays impinging on the fluorescent material on the mailthrough the opening 44 having their maximum of energy in the regionbetween 300 and 400 millimicrons. The rays emitted by the fluorescentmarks have a substantial part of energy at wave lengths greaterthan 550*millimicrons and, accordingly, are quite readily detected by thedetecting apparatus 60, since the combination of the detecting apparatus60 :and the filter 51 is sensitive just within this spectral region asexplained above.

It will be appreciated that the combination of the above features,namely, the low-pressure mercury vapor lamp and the particular filterand detecting arrangements, enables fluorescent markings to be usedwhich may be either transparent or translucent. Furthermore, thecombination enables the apparatus to differentiate between the actualfluorescent markings used for directing the mail to be sorted out andthe fluorescent properties of the paper itself.

In a more elaborate version of the above-described apparatus, twoseparate detectors could be used, one of which could be trained onto aportion or" the mail which normally does not carry any fluorescentmarking and the other could be trained onto a portion of the mail whichdoes carry such markings. The response of the two detectors may then becompared with one another so that the difference between the responsedue to the fluorescent markings and the response due to the inherentfluorescent property of the paper could be even more clearlydiflerentiated.

It will be appreciated that the present invention can also be used tosense the position of a postage stamp, provided the latter carries asuitable fluorescent material. Thus, a letter or postal card which,usually, carries a stamp at the upper right-hand corner, canautomatically be placed in the proper upright position and, thereafter,be processed through a mail routing apparatus.

It will be under-stood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

We claim:

1. In a method for automatically sorting mail, the steps of: depositinga fluorescent material on the mail to be sorted, said material havingsuch properties that a substantial portion of the energy emittedtherefrom upon excitation includes rays having wave lengths larger than550 millimicrons; producing, along a preselected path, exciting rays forinducing fluoresence of said fluoracent material, substantially all ofsaid exciting rays having wave lengths in the region between 300 and 400millimicrons; moving the mail to be sorted into said preselected path sothat said exciting rays impinge on said fluorescent material; anddetecting any rays emitted from said fluorescent material uponexcitation which have wave lengths larger than 550 millimicrons.

2. In :a method for automatically sorting mail, the steps of: depositinga fluorescent material on the mail to be sorted, said material havingsuch properties that a substantial portion of the energy emittedtherefrom upon excitation includes rays having wave lengths larger than550 millimicrons; producing rays having wave lengths of substantially254 millimicrons; transforming said rays into exciting rays for inducingfluorescence of said fluorescent material, said exciting rays havingwave lengths in the region between 300 and 400 millimicrons and beingdirected towards a preselected path; moving the mail to be sorted intosaid preselected path so that said exciting rays impinge on saidfluorescent material; and detecting any rays emitted from saidfluorescent material upon excitation which have wave lengths larger than550 millimicrons.

3. Method as defined in claim 1 wherein said fluorescent material isplaced on postage stamps gummed to the mail to be sorted.

4. In an apparatus for automatically sorting mail; wherein the mail ismarked with a fluorescent material having such properties that asubstantial portion of the energy emitted therefrom upon excitationincludes rays having wave lengths larger than 550 millimicrons, thecombination which comprises: means for producing exciting rays for thefluorescent material along a preselected path, substantially all of saidexciting rays having wave lengths in the region between 300 and 400millimicrons; means for moving the mail to be sorted into saidpreselected path so that said exciting rays impinge on the fluorescentmaterial; and means for detecting any rays emitted from said fluorescentmaterial due to the impingement of said exciting rays thereon, saiddetecting means being responsive only to rays longer than 550millimicrons.

5. In an apparatus for automatically sorting mail, wherein the mail ismarked with a fluorescent material emitting rays, upon excitation,having wave lengths longer than 550 millimicrons, the combination whichcomprises: means for moving the mail to be sorted along a preselectedpath; means spaced from said path for producing rays having wave lengthsof substantially 254 millimicrons; transforming means arranged betweensaid ray producing means and said preselected path for transforming saidrays into exciting rays and directing said exciting rays towards saidpath so as to impinge on the fluorescent material marked on the mail,substantially all of said exciting rays having wave lengths larger than254 millimicrons; filter means arranged between said transforming meansand said preselected path, said filter means having a high transmissioncharacteristic for rays having wave lengths in the region between 300and 400 millimicrons and having a low transmission characteristic forall other rays; and detecting means for detecting any rays emitted fromsaid fluorescent material due to the impingement of said exciting raysthereon, said detecting means having a broad receiving spectrum theshort-wave limit of which is approximately 550 millimicrons.

6. In an apparatus for automatically sorting mail, wherein the mail ismarked with a fluorescent material emitting rays, upon excitation,having wave lengths longer than 550 millimicrons, the combination whichcomprises: means for moving the mail to be sorted along a preselectedpath; a low-pressure mercury vapor source for producing rays having:wave lengths of substantially 254 millimicrons; transforming meansarranged between said source and said preselected path 'for transformingsaid rays into exciting rays and directing said exciting rays towardssaid path so as to impinge on the fluorescent material marked on themail, substantially all of said exciting rays having wave length largerthan 254 millimicrons; filter means arranged between said transformingmeans and said preselected path, said filter means having a hightransmission characteristic for rays having wave lengths in the regionbetween 300 and 400 millimicrons and having a low transmissioncharacteristic for all other rays; and detecting means for detecting anyrays emitted from said fluorescent material due to the impingement ofsaid exciting rays thereon, said detecting means having a broadreceiving spectrum the shortwave limit of which is approximately 550millimicrons.

7. Apparatus as defined in claim 6 wherein said transforming means are afluorescent layer arranged on said low-pressure mercury vapor source.

8. Apparatus as defined in claim 6 wherein said transforming means are afluorescent layer arranged on a carrier which is situated between saidfilter and said lowpressure mercury vapor source and is transparent toultraviolet rays.

9. Apparatus as defined in claim 6 wherein said transforming means are afluorescent layer arranged on said filter between said filter and saidlow-pressure vapor source.

10. Apparatus as defined in claim 6 wherein said detecting means areprovided with a filter having a pass range for rays having wave lengthslonger than 550 millimicrons.

11. Apparatus as defined in claim 6 wherein said lowpressure mercuryvapor source is U-shaped and said detecting apparatus are arrangedbetween the legs of the U.

12. Apparatus as defined in claim 11 wherein a plurality of detectingapparatus means are arranged between the legs of the U for detecting aseries of fluorescent marks on the mail to be sorted.

13. Apparatus as defined in claim 12 wherein said plurality of detectingapparatus are separated from one another by means of a plurality ofparallel partitions.

14. Apparatus as defined in claim 13 wherein said partitions areprovided with reflecting surface portions.

15. Apparatus as defined in claim 6 wherein at least a part of the spacebetween said preselected path and said detecting means constitutes a raycondensing member.

References Cited in the file of this patent UNITED STATES PATENTS2,200,853 Porter et al. May 14, 1940 2,350,001 Van den Akker May 30,1944 2,551,650 Urbach May 8, 1951 2,565,151 Taylor Aug. 21, 19512,609,928 Doust Sept. 9, 1952 2,950,799 Timms Aug. 30, 1960

5. IN AN APPARATUS FOR AUTOMATICALLY SORTING MAIL, WHEREIN THE MAIL ISMARKED WITH A FLURESCENT MATERIAL EMITTING RAYS, UPON EXCITATION, HAVINGWAVE LENGTHS LONGER THAN 550 MILLIMICRONS, THE COMBINATION WHICHCOMPRISES: MEANS FOR MOVING THE MAIL TO BE SORTED ALONG A PRESELECTEDPATH; MEANS SPACED FROM SAID PATH FOR PRODUCING RAYS HAVING WAVE LENGTHSOF SUBSTANTIALLY 254 MILLIMICRONS; TRANSFORMING MEANS ARRANGED BETWEENSAID RAY PRODUCING MEANS AND SAID PRESELECTED PATH FOR TRANSFORMING SAIDRAYS INTO EXCITING RAYS AND DIRECTING SAID EXCITING RAYS TOWARDS SAIDPATH SO AS TO IMPINGE ON THE FLUORESCENT MATERIAL MARKED ON THE MAIL,SUBSTANTIALLY ALL OF SAID EXCITING RAYS HAVING WAVE LENGTHS LARGER THAN254 MILLIMICRONS; FILTER MEANS ARRANGED BETWEEN SAID TRANSFORMING MEANSAND SAID PRESELECTED PATH, SAID FILTER MEANS HAVING A HIGH TRANSMISSIONCHARACTERISTIC FOR RAYS HAVING WAVE LENGTHS IN THE REGION BETWEEN 300AND 400 MILLIMICRONS AND HAVING A LOW TRANSMISSION CHARACTERISTIC FORALL OTHER RAYS; AND DETECTING MEANS FOR DETECTING ANY RAYS EMITTED FROMSAID FLOURESCENT MATERIAL DUE TO THE IMPINGEMENT OF SAID EXCITING RAYSTHEREON, SAID DETECTING MEANS HAVING A BROAD RECEIVING SPECTRUM THESHORT-WAVE LIMIT OF WHICH IS APPROXIMATELY 550 MILLIMICRONS.